For detecting a speed of movement of a digital camera lens, a photo interrupter is used. The photo interrupter here employs a light-receiving element having two terminals. Further, as the light-receiving element of the photo interrupter, a phototransistor has conventionally been used.
FIG. 7 shows a configuration of a conventional light-receiving element employing a phototransistor. When light enters this light-receiving element, a current flows between an output detection terminal and GND. This current is a photocurrent amplified hfe times. As a result, a potential difference is produced at an external resistor section. Due to this potential difference, a voltage of the output detection terminal decreases and consequently, light detection becomes possible.
Patent Literature 1 discloses to widen a range of an output voltage by reversal of an application bias voltage in an optical sensor including a light-receiving element having two terminals. Patent Literature 2 discloses a technique for making it possible to detect slight light to intense light by providing a switch in a circuit for amplifying a photocurrent caused by light received, in an optical sensor including a light-receiving element having two terminals. Further, Patent Literature 3 discloses to obtain a semiconductor device capable of detecting a low illuminance in an optical sensor including a light-receiving element having two terminals.
Patent Literature 4 discloses a device for detecting a rotational speed. This device employs an SMR element or an MR element. In this device, a sensor means including a detection element (detecting section) is driven by a constant voltage that is generated in constant voltage generating means. Then, an output of the sensor means is compared with a comparative voltage by comparison means. Further, in the device, a high current value and a low current value of a square wave current are switched by on and off outputs of a comparator in the comparison means. This device detects a physical quantity of a magnetic field. However, this device may be used as an optical sensor having two terminals, by replacing the detection element with a photodiode.